The present invention relates to a capacitor foil of aluminum or an aluminum alloy having edges created by a LASER beam.
Capacitors for energy storage purposes normally comprise a plurality of windings 10 which, depending on the application, are connected either in series or in parallel. FIG. 1 represents a schemetical cut through a winding. Voltage is applied between the two electrodes E1 and E2. A dielectric material 12 separates the two electrodes and defines together with an insulating fluid the dielectric behavior of the capacitor. Generally, the insulating fluid impregnates and surrounds the whole capacitor winding.
In the past, capacitors were produced with paper as the dielectric material. As reported in the paper "On The Characteristics of Aluminum Oxide-Loaded Capacitor Paper" by M. Tuuri et al., CIGRE Session 1962, Report No. 110, Dec. 17, 1962, investigations have been carried out into the effect of aluminum oxide added to capacitor paper. These investigations have shown that the power factor, the insulation resistance, and the life of the aluminum oxide-loaded paper were better than those of paper without aluminum oxide. No noteworthy differences were found in the dielectric constant and breakdown voltage. The tests also indicated that the change in the power factor of aluminum oxide-loaded paper under stress was less severe than the corresponding change in paper with no aluminum oxide.
In a typical, modern capacitor dielectric of All-Film technology, the dielectric comprises one or more layers, typically two, of plastic foil and a saturation or impregnation fluid. Paper, whether embedded with aluminum oxide or not, is no longer employed. These modern capacitors have an average field strength (effective value) of 50 V/.mu.m. At the edges of the foil forming the windings, there is theoretically a high field strength of unlimited magnitude, although actual field strength at the edges can neither be calculated nor measured.
A pure aluminum foil such as double rolled pure aluminum foil is generally used as an electrode for alternating-current capacitors in which the winding effective voltage lies above 600 V. At winding voltages under this value, an aluminum or zinc layer evaporated on the plastic film dielectric or a layer of a mixture of these two metal functions is used as the electrode unless special requirements are made of the capacitor. Special requirements are for example good capacitance stability, large temperature range and long life.
Field calculations show that the field strength increase at the electrode edges is smallest when all the electrode edges are exactly one above the other. Winding of electrode edges exactly one above the other is a condition which usually cannot be fulfilled in production. This is due to various causes such as, for example, the width tolerance of the electrodes, typically larger than 0.2 mm, and the mechanical stresses in the plastic films used for the dielectric media. The stresses present in the plastic films often cause the films and hence the electrodes to be offset laterally during the winding process. With a typical dielectric thickness of 16 to 30 .mu.m, the edges would have to be wound exactly above one another in an area of only a few .mu.m to be able to speak of edges over one another. If the electrodes, which are at a different potential, are laterally offset only by even the amount of thickness of the dielectric, this results in an increase of the field strength at the inner edge which almost corresponds to the final value of the field strength which occurs with a large offset.
Using the above knowledge, electrodes have been dimensioned and wound in such a way, that they protrude from the winding on one side, each one opposite the other. This generally does not result in additional weak points. This production method facilitates contact-making and also has advantages for the overall behavior of the capacitor. In comparison with traditional contact-making by means of contact tags, this results for example in a reduction of the connection inductivity. The locally occurring, maximum current density is also lower. In addition there are no contact tags that could cause local damage to the dielectric.
In order to utilize the dielectric better, the inner electrode edge can be folded over. If the beginning and the end of the electrode are also folded over, then there are no sharp edges either in the inside or at the edge of the dielectric. The improvement in the dielectric properties of a capacitor produced in this manner is remarkable. An undesired effect is, however, the thickening of the winding caused by the folding over. In order to be able to impregnate such windings correctly with an insulating oil, the winding must be relatively loose. This has a negative effect on the stability both of the winding and the winding stack.
It is believed that if the inner edge can be rounded off without causing a local thickening of the electrode which hinders the impregnation of the winding, then further improvements in the dielectric behavior of the capacitors, as compared to capacitors with folded-over electrode edges, can be expected. One method of producing rounded-off electrode edges consists of cutting the capacitor foil by means of a LASER. Such a capacitor foil is already known through Japanese Patent Application 55-030813.
By cutting with a LASER beam, the cut edge of the metal foil is rounded with the result that there is a significant diminution in the burrs and cracks typically formed at the edge of the metal foil when cutting with mechanical cutting tools, in particular circular tools for that purpose. Such burrs and cracks lower the maximum nominal voltages available from capacitors using such foils because of the local increase in field strength at these sites. The increase in field strength places a heavy local demand on the dielectric medium which leads to premature failure of the capacitor and which renders it impossible to make optimum use of the capacitor dielectric.
Since aluminum surfaces strongly reflect LASER light, it is not possible to separate aluminum foil in one continuous cut. The LASER used must be run in an intermitting mode to supply the energy density required for melting. This results in a sawtooth-like formation of the cut edge. Although each one of these "teeth" is rounded off, without special measures the irregular formation of the edge does not lead to the required improvement of the dielectric behavior of a capacitor made with a foil of this kind.
The thermal nature of the LASER process creates in its area of influence not only a rounded off cut edge, but also leads to the formation of an oxide layer on the edge and in its immediate vicinity. Hitherto this oxide layer has not been taken into account in the observation of the properties of a power capacitor for alternating current. As will be seen from the following discussion, it can, however, play an important role in the overall system even though the formation of the oxide layer is difficult to control.